Sound translating device



Jun 29, 1937. L. G. BOSTWICK 2,085,073

SOUND TRANSLATING DEVICE Filed Feb. 21, 1935 RADIATION RES/STANCE A TDIAPHRAGM '5' 2 IFII?EOIUE}VCII/ INVENTOR By L. a. 50s TW/CK Wait, '6.7M

A T TOR/VEV Patented June 29, 1937 ersr QFFiiYZE SOUND TRANSLATINGDEVICE Application February 21, 1935, Serial No. 7,484

13 filaiims.

This invention relates to sound translating devices and moreparticularly to loud-speakers of the moving coil type including adiaphragm of sufficient area to propagate sound waves of substantialintensity without the aid of a horn.

In loud-speakers including relatively large direct acting pistondiaphragms of the mass controlled type, that is loud-speakers whereinthe mass reactance of the vibrating system is made the predominatingvibratory impedance, substantially constant electro-acoustic conversionefiiciency may be obtained throughout the lower portion of theaudio-frequency range. Throughout this range the vibrational velocity ofthe diaphragm varies inversely as the frequency and the radiationresistance varies directly as the square of the frequency. The range offrequencies in which these relations obtain is dependent upon the radiusof the diaphragm, and in general, the

larger the diaphragm, the smaller is the frequency range wherein theacoustic power is substantially uniform. If it were desired to attain auniform output from a device of this type throughout a frequency rangeof several. thousand cycles, the

diaphragm necessarily would be so small that an unduly large vibrationalamplitude would be required for even moderate acoustic outputs. If alarger diaphragm were used to avoid these difficulties, the acousticoutput at the higher frequencies would be deficient and an undesirableconcentration of the sound field at high frequencies would result.

Furthermore, loud-speakers having mass-controlled diaphragms, ingeneral, inherently have a relatively low electro-acoustic efficiency ascompared, for example, with horn type loud-speakers. This is occasionedby the fact that inasmuch as the mass reacta'nce of the vibrating systemin such loud-speakers is the predominating imped- 4O ance, a largeportion of the actuating force for the vibrating system is expended inaccelerating the vibrating mass so that but a relatively small portionthereof is available for producing sound radiations.

45 One general object of this invention is to reproduce sound uniformlemciently and with substantial intensity throughout a relatively wideband of frequencies. More specifically, objects of this invention are:

To obtain with loud-speakers having diaphragms sumciently large topropagate sound waves of substantial intensity without the aid of ahorn, the relatively wide frequency range of uniform reproduction, andthe wide sound field 55 obtainable with nailer diaphragms;

(Cl. 18l-31) To obtain, with hornless loud-speakers, a highelectro-acoustic efficiency throughout a wide band of frequencies;

To prevent, in loud-speakers having chambers adjacent the non-radiatingsurface of the diaphragm, undesirable resonance in such chambers.

In one embodiment illustrative of this invention, a loud-speakercomprises a magnet having pole-pieces spaced to form an air-gap, the gapbeing sealed at one end by a gasket. A diaphragm is mounted upon themagnet and includes a peripheral portion. secured upon the outerpolepiece, a central dished or dome-shaped portion, and an annulartroughed intermediate portion connecting the peripheral and the dishedor dome-shaped portions. The diaphragm is actuated by a coil disposed inthe air-gap and preferably is of such construction that the centralportion vibrates as a whole, analogous to a piston, throughout the bandof frequencies to be reproduced.

In accordance with a feature of this invention, a dished or dome-shapedmember is provided adjacent the outer or radiating surface of the dishedor dome-shaped portion of the diaphragm, this member being provided withsound openings. The openings are made of such area that the radiationresistance at the diaphragm varies as the square of the frequency up tosubstantially the highest frequency it is desired to reproduce.

In accordance with another feature of this invention, the openings andthe chamber between the dished or dome-shaped portion of the diaphragmand the dome-shaped member are so designed that the mass reactance ofthe air at the openings and the stiffness reactance of the chamber aresubstantially equal at or near the highest frequency in the band it isdesired to reproduce. This accentuates the radiation resistance at thediaphragm at the higher frequencies of the band reproduced, therebyincreasing the acoustic output. This increase may correspond to asimilar increase at the lower frequencies in the band reproduced,attained in other ways, such for example, as described in my patents1,895,441, granted January 31, 1932 and 1,967,223, granted July 24,1934, whereby the efficiency of the device is effectively increased.

In accordance with a further feature of this invention, the chambersbehind the dished or dome-shaped and troughed portions of the diaphragmare designed so that the pressure changes in these chambers are equaland in phase. Air flow through the air-gap from one chamber to theother, therefore, does not occur and undesirable resonance effects areavoided.

The openings in the dished or dome-shaped member may be of various formsand may be disposed in various ways, the form and disposition being soChosen that the sound field distribution is particularly suitable tomeetthe exigencies of the specific installation in which the loudspeaker isutilized. It is usually desirable, however, that the openings be sodisposed that substantially uniform sound. paths are obtained for Wavesoriginating at spaced points on the diaphragm whereby interferenceeffects are reduced.

The invention and the several features thereof will be understood moreclearly and fully from the following detailed description with referenceto the accompanying drawing in which:

Fi l is a side elevational view, partly in crosssection of aloud-speaker constructed in accordance with, and illustrative of, oneembodiment of this invention;

Fig. 2 is a top view of the dished or domeshaped member overlying thediaphragm in the loud-speaker shown in Fig. 1;

Fig. 3 is a top view, to a reduced scale, of another dished ordome-shaped member which may be incorporated in a loud-speaker of thegeneral construction shown in Fig. l; and

Fig. 42 illustrates typical radiation resistance characteristics of aloud-speaker of usual design including a relatively large diaphragmradiating directly into the atmosphere and of a loudspeaker, such asshown in Fig. 1, constructed in accordance with this invention.

Referring now to the drawing, the loud-speaker in Fig. 1 comprises amagnet having a central pole it? encompassed by a field or excitationcoil ii, and an outer pole l2. The central pole has secured thereto, asby screws l3, only one of which is shown, a circular pole-piece i l, andthe outer pole if has secured thereto, as by screws l5, an annularpole-piece it. The pole-pieces i i and it are coaxiaily arranged andspaced to form an annular air-gap it, the inner end of the gap beingacoustically sealed by an annulus l8, which, for example, may be ofrubber and suitably secured to the pole-pieces i l and it.

A diaphragm, which may be of a suitable lightweight metal such asduralumin, is mounted upon the outer pole-piece l6 and includes a dishedor dome-shaped portion ill, a peripheral portion 2t and an intermediateannular flexible troughed portion The diaphragm may be fabricated in onepiece or the several portions may be formed separately of the same ordifferent materials. The peripheral portion ff: is clamped between tworings 22 and 23 which are held together and secured to the outerpole-piece it by screws 24. A cylindrical form or support which may beof impregnated paper, is suitably secured to the diaphragm, bycementing, adjacent the base of the dished portion ill and carries anannular driving coil 26 which is disposed in the air-gap ll. As will beseen from Fig. l, the dished portion of the diaphragm, the form orsupport 25 and the pole-piece i l form a relatively large chamber 2?,having a volume V1 and stiffness S1, and the annular troughed portion 2!the form or support 25, ring 23, and pole-piece l6 form a relativelysmall chamber having a volume V2 and stiffness S2, the two chambers Eland 28 being serially connected by the air-gap ll. The troughed portionfit is preferably made sufliciently flexible and the dished portion i8is preferably made sufficiently rigid so that the dished portion iii mayvibrate bodily analogous to a piston through the range of frequenciesdesired to be reproduced.

As is known in the art, the radiation resistance of a diaphragmvibrating as a piston in open air varies approximately as the square ofthe frequency up to some frequency such as f1 as shown in Fig. 4. Abovethis frequency the radiation resistance increases less rapidly andfinally approaches a substantially constant value after going through aseries of oscillations as shown by curve A in Fig. 4. If the vibrationalvelocity of the diaphragm is inversely proportional to the frequency, asis the case for mass controlled diaphragms, the sound radiated will beuniform up to the frequency f1 (Fig. 4) but above this frequency thesound radiated will decrease in correspondence with the less rapidincrease in the radiation resistance.

The frequency range throughout which the radiation resistance isproportional to the square of the frequency is dependent upon the sizeof the diaphragm. In order that the radiation resistance will varyapproximately as the square of frequency up to a certain desiredfrequency, the radius of the diaphragm must not be appreciably greaterthan that given by the relation where Ta is the radius of the diaphragm,and i is the desired frequency.

From Equation (1), it will be apparent that in order to obtain uniformacoustic output throughout a range of several thousand cycles from aloud-speaker having a mass controlled diaphragm, the diaphragm wouldhave to be small. Inasmuch as the radiation resistance is dependent uponthe size of the diaphragm, the acoustic output from a diaphragm of thesmall size necessitated by the above relation would be of smallmagnitude unless, as has been noted hereinbefore, the amplitude ofvibration of the diaphragm were relatively great. For example, if itwere desired that the acoustic output be uniform up to 8000 cycles, theradius of the diaphragm would have to be not materially greater thaninch; and in order to obtain an acoustic output of approximately twowatts at 400 cycles it would be necessary that a diaphragm of this sizehave a double vibration amplitude of about two inches. A loudspeakerhaving diaphragm of this size and vibrating with this amplitude would beimpractical, for the diaphragm suspension means permitting such anamplitude would be extremely difficult to obtain and the cost of themagnet would be relatively high.

From the foregoing, it would follow, then, that a diaphragm larger thanthat permitted by Equation (1) must be used if a substantial acousticoutput of the order of the magnitude indicated above is desired and theresulting nonuniformity of response permitted. For example, in order toobtain a substantial output without unduly large vibrational amplitudesof the diaphragm and to obviate the use of a relatively costly magnet, adiaphragm having a diameter of four inches has been used. For adiaphragm of this size, as determined from Equation (1), f1 would beabout 780 cycles.

In accordance with this invention, the limitations set forth hereinaboveare circumvented, so that a relatively greater acoustic output isobtained and the range of frequencies throughout which the output willbe uniform is materially r c. g. s. units increased. In one illustrativeembodiment of this invention shown on Fig. 1, a diaphragm of a sizelarger than that dictated by Equation (1) is used, so that greateracoustic output is obtained, and a rigid dished member or cover 29 isplaced over the outer or radiating surface of the diaphragm. This dishedmember or cover conforms in shape to the central portion I9 of thediaphragm and forms therewith a chamber 30 having a volume V3 andstiffness S3. The cover is secured upon the ring 22 by the screws 24 andis provided with a plurality of elongated parallel openings or aperturesSt. The aggregate area of the elongated apertures 3! is made equal tothat of a circle with a radius sufiiciently small to approximatelysatisfy Equation (1) at the highest frequency it is desired toreproduce. For example, if it is desired that the acoustic output besubstantially uniform up to 3,000 cycles (f2 in Fig. 4) the aggregatearea of the openings could be of the order of five square centimeters.

The radiation resistance, RR, at the openings 3| may be expressed by theequation c. g. s. units I I' 60 11 QI, 1s 10 (3) From Equations 1, 2 and3, it will be seen that the air load on the diaphragm of a loud-speakerconstructed in accordance with this invention is the same as the airload on the diaphragm of a loud-speaker without the cover, up to thelimiting frequency 1 in Fig. 4) for the diaphragm of radius Td asdictated by Equation (1). However, in a loud-speaker constructed inaccordance with this invention, the air load increases as the square offrequency up to a frequency (f2 in Fig. 3) determined by the radius To,so that a substantial increase in the frequency range throughout whichuniform output obtains is achieved. Inasmuch as the air load on thediaphragm is the same for frequencies up to 1 for loud-speakers with orwithout a cover, such as the cover 29, the efiiciency and sound powercapacity of a loud-speaker constructed in accordance with this inventionis as great as that of loud-speakers without a cover, with which auniform output is obtainable up to the frequency f1, and is greater thanthat of loudspeakers without a cover, with which a uniform output isobtainable up to the frequency f2.

In order to obtain a relatively high electroacoustic conversionefiiciency in devices of the type contemplated by this invention, theresonance frequency of the vibrating system should be within the band offrequencies it is desired to translate, as described in detail in myaforesaid Patents Nos. 1,895,441 and 1,967,223. In the embodiment of myinvention illustrated in Fig. 1, the resonance frequency of thevibrating system is dependent upon, among other factors, the parametersof the diaphragm and of the chambers 21 and 28. If, for example, it isdesired that the range of frequencies to be reproduced be 400 to 3,000cycles, the diaphragm and chambers 27 and 28 may be so designed that theresonance frequency of the vibrating system is approximately 500 cycles.

This increases the acoustic output at the lower frequencies andnecessitates some further accentuation of the higher frequencies inorder that the acoustic output be substantially uniform throughout thedesired band of frequencies. This accentuation may be accomplished by sodesigning the member 29 that the stiffness reactance of the chamber 36is equal to the air mass reactance, wMR, of the openings 3|, at or nearthe frequency f2. This produces an anti-resonance effective at thediaphragm, which results in an increase in both the radiation resistanceand the velocity of the vibrating system at the higher frequencies, thatis, the frequencies in the vicinity of f2.

The air mass reactance, 60MB, of the openings 3! may be ascertained fromthe relation fl d l y) where p is the air density c being the velocityof sound, K1(y) is a Bessel function represented by the series Thestiffness of the chamber 3|! may be ascertained from the relation whereAd is the effective area of the diaphragm and the remaining charactershave the same significance as hereinbefore.

As noted previously, the openings in the dished or cover member 29 maybe of various forms and disposed in various ways. As shown in Fig. 3,for example, the cover may be provided with arcuate openings 32 disposedin circular formation about the center of the cover. A single straightslit or opening, or opening of other form may be used and advantagetaken of the sound field distribution characteristics of the particu larform of opening. The specific forms and arrangements of the openingsshown in Figs. 2 and 3 have the particular advantages of equalizing thesound paths between various parts of the dished portion l9 of thediaphragm and the openings whereby interference effects aresubstantially eliminated, and of insuring a substantially uniformdistribution of all frequencies in the band radiated throughout arelatively large solid angle.

In order to prevent local resonance effects to the rear of thediaphragm, the chambers 21 and 28 are so designed that the pressurechanges in these chambers, occasioned by vibration of the diaphragm, areequal and in phase so that air flow through the air-gap I! from onechamber to the other does not occur. The pressure changes in thechambers 2'! and 28 are equal when the chambers are so proportioned thatL LE V1 V2 where V1 and V2 are the volumes of the chambers 2! and 28,respectively, when the diaphragm is at rest, and 6V1 and 6V2 are thechanges in volumes in the chambers 21 and 28, respective- 1y, occasionedby vibrational displacement of the diaphragm.

Although specific embodiments of the invention have been shown anddescribed, it is to be understood, of course,that modifications may bemade therein Without departing from the scope and spirit of thisinvention as defined in the appended claims.

What is claimed is:

1. A sound translating device comprising a diaphragm having a portionvibratile as a whole throughout a band of audio frequencies, saidportion being of sufficient area to propagate sound waves of substantialintensity without the aid of a horn, means for actuating said diaphragm,and means adjacent the radiating surface of the diaphragm for increasingthe range of frequencies in which the radiation resistance at saiddiaphragm varies as the square of frequency, by several thousand cycles.

2. A sound translating device comprising a direct acting diaphragmhaving a portion of the order of four inches in diameter vibratile as awhole throughout a band of audio frequencies, means for actuating saiddiaphragm, and means adjacent the radiating surface of said diaphragmfor increasing the range in which the radiation resistance at saiddiaphragm varies as the square of frequency up to substantially 3000cycles.

3. A sound translating device comprising a direct acting diaphragmhaving a portion vibratile as a Whole throughout a band of audiofrequencies, said portion having a radiating surface in communicationwith the atmosphere and the opposite surface acoustically sealed fromthe atmosphere, and a member adjacent said radiating surface and forminga chamber therewith, said member having a restricted sound opening ofsuch area that the radiation resistance at said diaphragm varies as thesquare of frequency up to substantially the highest frequency saiddevice is intended to translate.

4. A loud-speaker comprising a diaphragm having a portion vibratile as awhole throughout a band of audio frequencies, said diaphragm being ofsufficient area to propagate sound Waves of substantial intensitywithout the aid of a horn, means for actuating said diaphragm, and amember overlying said portion and forming a chamber therewith, saidmember having a restricted sound opening of an area commensurate withthe area of a mass-controlled piston type diaphragm for which theradiation resistance varies substantially as the square of frequency upto the highest frequency said first diaphragm is intended to radiate.

5. A sound translating device comprising a diaphragm having adome-shaped portion, means for supporting said portion so that it isvibratile as a whole, means for actuating said diaphrgam, and means forincreasing the range in which the radiation resistance of said diaphragmvaries substantially as the square of frequency, said third meansincluding a dome-shaped member overlying said dome-shaped portion andhaving a restricted opening therein.

6. A loud-speaker comprising a diaphragm of sufficient area to propagatesound waves of substantial intensity without the aid of a horn, saiddiaphragm including a dished portion and a supporting portionsufficiently flexible to allow said dished portion to vibrate as awhole, means for actuating said diaphragm, and means for increasing therange in which the radiation resistance at said diaphragm variessubstantially as the square of frequency, including a dome-shaped membersubstantially conforming to said dished portion, said member having aplurality of restricted openings.

7. A loud-speaker comprising a direct acting diaphragm having a dishedportion of sufficient area to propagate sound waves of substantialintensity without the aid of a horn, said portion having a radiatingsurface in direct communication with the atmosphere, means sealing theopposite surface of said portion from the atmosphere, means supportingsaid dished portion so that it is vibratile as a whole, means foractuating said diaphragm, and a dished member overlying the radiatingsurface of said dished portion and having a plurality of elongatedparallel openings.

8. A loud-speaker comprising a direct acting diaphragm having a dishedportion vibratile as a whole and of suflicient area to propagate soundwaves of substantial intensity Without the aid of a horn, said portionhaving its radiating surface in direct communication with theatmosphere, means for actuating said diaphragm, and a dished memberoverlying the radiating surface of said dished portion and having aplurality of arcuate openings arranged in circular formation about thecenter of said dished member.

9. A sound translating device comprising a diaphragm having adome-shaped portion, and a dome-shaped member overlying said portion andforming a chamber therewith, said member having sound openings thereinof an area such that the air mass reactance at said openings and thestiffness reactance of said chamber are substantially equal at thehighest frequency said device is intended to translate.

10. A loud-speaker comprising a diaphragm of suflicient area topropagate sound waves of substantial intensity without the aid of ahorn, said diaphragm having a dished radiating portion, means foractuating said diaphragm, and a domeshaped member adjacent andconforming to said dished portion and forming a chamber therewith, saidmember having a plurality of elongated openings of such area that theair mass reactance at said openings is substantially equal to thestiffness reactance of said chamber at the highest frequency saidloud-speaker is intended to reproduce.

11. A loud-speaker comprising a direct acting diaphragm having a portionof the order of four inches in diameter and vibratile as a whole, meansfor actuating said diaphragm, and a member adjacent the radiatingsurface of said portion having one or more sound openings therein ofsuch aggregate area that the radiation resistance at said diaphragmvaries as the square of frequency up to substantially 3000 cycles.

12. A loud-speaker comprising a direct acting diaphragm having a dishedportion of the order of four inches in diameter and vibratile as awhole, means for actuating said diaphragm, and a dished member overlyingthe radiating surface of said dished portion and substantiallyconforming thereto, said dished member having one or more openingstherein of an aggregate area commensurate with the area of amass-controlled piston type diaphragm for which the radiation resistancevaries as the square of frequency up to substantially 3000 cycles.

13. A loud-speaker comprising a diaphragm having a dome-shaped portionof the order of four inches in diameter and vibratile as a whole, meansfor actuating said diaphragm, and a dome-shaped member overlying saiddome-shaped portion and forming a chamber therewith, said member havingsound openings of an aggregate area commensurate with the area of amass-controlled piston type diaphragm for which the radiation resistancevaries as the square of frequency up to substantially 3000 cycles, saidopenings and said chamber being so constructed that the mass reactanceof the air at said openings and the stiffness reactance of said chamberare substantially equal at 3000 cycles.

LEE G. BOSTWICK.

